Analysis of different self-propulsion types of oil droplets based on electrostatic interaction effects

This study aims to investigate electrical effects in self-propelled oil droplets and analyze the motion caused by electrical interactions between two droplets. We examine the motion of self-propelled oil droplets in terms of electrical effects; no similar analyses have been reported to date. When an oleic acid droplet is introduced into a surfactant solution, oleic acid is adsorbed at the droplet surface; as a result, the surface becomes negatively charged. However, the electrostatic interaction between oil droplets has not been studied so far. We focus on the chargeability of self-propelled oil droplets, and propose that their motion can be controlled by changing the charge state at the oil/water interface. This is realized by controlling the pH of the surfactant solution. We prepared several aqueous solutions with slightly different pH values, and introduced two oil droplets into each solution to investigate the relationship between the pH value and the correlated motion of the oil droplets. In an aqueous solution at pH 12.00, the two oil droplets showed two types of motion (follow-up and parallel motion), while in an aqueous solution at pH 12.10 they exhibited only one type of motion, i.e., repulsive motion. In order to analyze the three types of motion, we investigated electrical effects on the surface of the oil droplets. First, we measured the zeta potentials of aqueous solutions at pH 12.00 and 12.10, and analyzed the relationship between pH and charge. Furthermore, we calculated the amount of charge and the repulsive force generated on the surface of the oil droplets. For the pH 12.10 solution, the charge of one oil droplet and the repulsive force generated by the charge were found to be -30.79 pC and 282 nN, respectively. Taking into account the driving forces controlling the motion of oil droplets reported in previous studies, this repulsive force could be the driving force of the observed repulsive motion. Based on the experimental results, we inferred that the three types of self-propulsive motion were controlled by electrical effects at the oil/water interface. To further examine these motions, we analyzed the time dependences of the total distance traveled by a droplet and of the distance between oil droplets, as well as the droplet velocities and images of internal convection of oil droplets.